Fluid couplers in the nature of torque converters are widely employed in vehicles to transfer torque between the engine and the transmission. In addition to this fundamental purpose, torque converters serve two other primary functions. First, they provide a means by which to effect a smooth coupling between the engine and the transmission. Second, they provide hydraulic torque multiplication when additional performance is desired.
The typical torque converter utilized in a vehicular drivetrain normally has three major components ---- viz.: a pump or input member (commonly designated as the impeller); the driven or output member (commonly designated as the turbine); and a reaction member (commonly designated as the stator).
Slots and tabs have been used, historically, to attach sheet metal impeller blades to the inner and outer housings. Including the cover as a component of the impeller assembly has simplified sealing the torque converter so that the impeller assembly may be contained within a housing that is filled with hydraulic fluid. The hydraulic fluid is circulated through the torque converter by rotation of the cover to which the impeller elements are affixed, and the circulating hydraulic fluid effects rotation of the turbine. The torque converter is thus a closed system with the impeller operatively connected to a source of input torque and with the turbine member operatively connected to deliver the output torque to the vehicular transmission.
Typically, the cover and the impeller elements that are attached thereto ---- which combine to constitute the impeller assembly ---- are affixed to a flex-plate that is bolted to the crankshaft of the engine. The turbine, on the other hand, is connected to an output shaft which exits the torque converter and becomes the input shaft of the vehicle transmission.
Because of the aforesaid mechanical connection between the engine and the impeller, the impeller will rotate at engine speed whenever the engine is operating. It is this rotation of the impeller which causes it to operate as a pump, and particularly a centrifugal pump. That is, the impeller ingests hydraulic fluid present at the central or hub portion thereof and discharges hydraulic fluid axially into the turbine at the radially outer rim of the impeller assembly. Whereas the impeller discharges the hydraulic fluid axially into the turbine, rotation of the impeller also imparts a circumferential or tangential component to the hydraulic fluid as it exits the impeller. As the hydraulic fluid exiting the impeller engages the turbine, the kinetic energy of the moving hydraulic fluid urges the turbine to rotate in response to rotation of the impeller.
When the vehicle is not moving, and even though the engine is idling, the impeller is not spinning sufficiently fast to supply the energy necessary to overcome the static inertia of the vehicle. In that situation, therefore, the hydraulic fluid simply flows through the turbine and the turbine does not rotate. This allows the vehicle to remain at rest, even though the transmission has been shifted into a selected drive range and the engine is running.
However, as the throttle is opened, the rotational speed of the engine, and therefore the impeller, increases. At some rotational speed of the engine, sufficient energy is being imparted to the turbine that it will be able to overcome the static inertia which prevents the vehicle from moving. At that time, the energy transferred from the impeller to the turbine will be delivered to the drive wheels through whichever of the drive ranges provided by the transmission has been selected.
Kinetic energy is most effectively imparted to the turbine when the hydraulic fluid circulating within the torque converter follows the contours of the turbine blades between the shell and core from which they are presented, and then leaves the turbine. However, the configuration of the turbine causes the fluid passing therethrough to exit in a direction that is generally inappropriate to that direction at which one would prefer to have the hydraulic fluid re-enter the impeller. Accordingly, were the fluid to re-enter the impeller in that direction, the fluid would strike the blades of the impeller in a direction that would be detrimental to the rotation of the impeller.
In order to minimize the problems resulting from the undesirable direction at which the fluid enters the impeller as it exits the turbine without any redirection, a stator is generally interposed within the path which the hydraulic fluid must traverse between its exit from the turbine and its re-entry into impeller. In fact, the stator redirects the hydraulic fluid which has exited the turbine so that the fluid will enter the input of the impeller in a direction that will cause the fluid to assist the engine in turning the impeller. The force which the hydraulic fluid thus imparts to the impeller comprises the source for the additional kinetic energy applied to the turbine. It is this additional energy applied to the impeller which results in an increase in the force applied to drive the turbine ---- thereby accomplishing torque multiplication.
Historically, the blades presented by the impeller have, for the most part, been stamped from sheet metal and then secured to the shell and core of the impeller. However, it must be understood that sheet metal blades are extremely sensitive to the angle of incidence ---- i.e.: the angular difference between the direction at which the hydraulic fluid actually flows into the impeller and the direction at which the fluid would most effectively engage the pump blades as they rotate. The angle of incidence is, therefore, at least partially a function of the disposition of the blades relative to the shell. In the typical torque converter heretofore described, the angle of flow incidence may vary through a range of approximately 75 degrees.
The incidence angle is equal to the difference between the inlet flow angle and the inlet blade angle. Incidence loss is due to the mismatch between the flow exit angle of one element and the inlet blade angle of the adjacent element. It occurs when the oil enters a blade row at some angle other than its physical inlet angle. The inlet flow angles changes with turbine/pump speed ratio. Since the physical blade angles are always at some fixed values, the torque converter elements operate under a wide range of incidence angle ---- the difference between the inlet flow and the inlet blade angle. Therefore, incidence loss is an important parameter to be considered in the design of the torque converter.
In some speed ratio conditions, incidence loss can be the dominant loss. For example, the inlet flow angle to a stator blade row may vary from -60 degrees at stall to +50 degrees at the coupling point. But, the inlet blade angle is a constant value of zero degree. Incidence angle is thus 60 and -50 degrees at stall and the coupling point, respectively. For an impeller or pump, incidence angle can be as high as -40 and +35 degrees at stall and the coupling point, respectively. For a turbine, incidence angle can be as high as +25 and -20 degrees at stall and the coupling point, respectively. Incidence loss is the dominant loss at stall.
One structural arrangement heretofore employed to delay stall is to fashion the blades as vanes having a hydrofoil configuration. It is well known that the bulbous-nosed curvilinear vanes having a hydrofoil configuration delay separation of the fluid from the vane. It should be understood that the fluid need not maintain laminar flow along the entire radial extent of the surface on both sides of the vane. In fact, it is highly likely that there will be boundary layer turbulence along the back surface of the vanes ---- i.e.: that surface which faces away from the direction in which the vanes are moving ---- but the boundary layer will nevertheless adhere to the vane along a much greater radial extent because of the hydrofoil configuration incorporated in the impeller vanes.
A definite improvement in flow characteristics is achieved along a greater portion of the radial extent of the impeller vanes by employing a hydrofoil configuration. Simply adopting the bulbous-nosed hydrofoil configuration at the input end of the vane also introduces certain hydraulic restrictions. Primarily, the increased cross section of the vane at the inlet of the impeller reduces the effective flow area and impeller flow capacity. On balance, however, greater advantages inure to the user of a hydrofoil configuration for the impeller vanes in comparison to the user of sheet metal blades. Nevertheless, simply adopting vanes having a hydrofoil configuration in place of the flat or curved sheet metal impeller blades falls short of the desiderata.